Protective system for furnaces



June 2, 1936. TL. ME'HARG ET AL 2,042,475 ROTECTIVE SYTEM FOR FURNACES Filed July 26, 1954 Stop Zimaentor laarezzce Mehag Jacob J7. jihylor' ,Donald I. Rode/M01 1} i a V Gttomem Patented June 2, 1936 UNITED STATES PATENT OFFICE PROTECTIVE SYSTEM FOR FURNACES Application July 26, 1934, Serial No. 737,132

31 Claims.

The invention relates to protective systems for furnaces particularly for forced draft, gas, oil or tar fired regenerative and recuperative furnaces, and while the invention will be specifically described in connection with forced draft regenerative furnaces, yet it will be apparent that the invention is equally well adaptable to forced draft recuperative furnaces. Accordingly, when regenerative furnaces are referred to herein and in the claims, it will be understood that it is intended to include recuperative furnaces.

Ordinary natural draft regenerative furnaces are so well known that any description of them is unnecessary; and the present invention does not relate to natural draft regenerative furnaces. To improve the efiiciency of the ordinary regenerative furnaces forced draft has been employed in place of natural draft; and this use of forced draft in regenerative furnaces brings with it entirely new problems. In forced draft regenerative furnaces a blower or blowers must be employed to create the draft, and motors are used to operate the blowers. In the particular forced draft operation illustrated, two short stacks of suflicient height to clear the building are employed; and each stack has a damper by means of which communication with the atmosphere can be shut off. These dampers can be operated by motor driven pumps, by compressed air, by solenoids, etc.

If there is any failure of the blower or blowers or any failure of the damper operations, if not immediately followed by a complete shut down of the furnace, the result might be quite serious. For example, in case of total power failure" the fan or fans Would be shut down, stopping the flow of combustion air and flue-gas ejecting air, while fuel would continue to flow into the furnace, until shut off by hand. This might result in an explosive mixture filling the furnace chamber and lines. Further, at the furnace operating temperature and with insufficient combustion air, a natural gas fuel would be cracked, with a resultant deposition of carbon on the surface of the molten glass or other substance in the furnace. The above mentioned conditions might arise with a total power failure when a single fan and motor or a plurality of fans and motors are operating; and the furnace operator would not be immediately aware of the condition unless he happened to be close to the furnace. If two 'or more fans and motors were operating and only one motor failed similar conditions would arise, and the furnace operator would have stillless notice of thecondition. 3

Failure of the mechanism which keeps one stack damper closed would result in both stacks ejecting from the furnace. This would cause cold air to be sucked into the furnace through every crack and crevice; and the fuel would be drawn down through the checkerwork to the stack. If sufficient air were sucked in with the fuel, combustion would take place in the checkers. If sufiicient air forcombustion were not sucked in, there is thepossibility of filling the checker chambers and fiues with an explosive mixture.

The foregoing will illustrate generally some of the undesirable and possibly dangerous conditions which might arise in the operation of forced draft regenerative or recuperative furnaces; which conditions do not occur in the operation of natural draft regenerative or recuperative furnaces. j

One of the objects of the present invention is to provide a protective system which will prevent the above-mentioned and other undesirable conditions from arising; and which will immediately give warning to the furnace operator of any total or partial power failure;

Another object of the invention is to provide such a protective system which will, in addition to giving warning to the operator, cause an immediate shut down of the furnace in the event of total or partial power failure.

A further object of the invention is to provide a protective system which will cause an immediate shut down of the furnace with warning to the operator, in the event of failure of any of the electrical equipment controlling the operation of the furnace. 7

Another object of the invention is to provide a protective system which will cause the immediate shut down of the furnace, with warning to the operator, in the event of some failure in the protective system itself.

Still another object of the invention is to provide aprotective system which in the event of a shutdown, will necessitate the starting of the furnace in a proper and definite sequence, 1. e., any deviation from the proper sequence will result either in the furnace not starting at all or in being immediately shut down again after starting. 7

Merely a few of the many advantages and objects of the invention are mentioned above. Briefly, the purpose is to prevent any unusual or undesirable conditions from arising in the furnace, due to total or partial power failure in the blower motor or motors, a total or partial power are operated by motors M, M.

failure in the damper controlling means, a failure in the motor circuit or circuits, a failure in the damper circuits, or a failure in the protective circuits, by automatically shutting down the furnace and giving warning to the operator. The many advantages of this protective system in the operation of forced draft regenerative or recuperative furnaces will be apparent to those skilled in the art, from the following description when taken in connection with the accompanying drawing, in which:

The figure illustrates diagrammatically one embodiment of the invention applied to a forced draft regenerative furnace.

Before describing the protective system it will be helpful in understanding the invention if a brief description is given of what may be considered as a typical control for a forced draft, gas-fired regenerative furnace. The control diagrammatically shown is, of course, merely illustrative of many different controls which could be employed with forced draft regenerative furnaces; and it should be mentioned here that controls for such furnaces were known prior to the present invention, and that the present invention does not in any way relate to any control per se but to a protective system for the controls.

Numeral 2| refers to a furnace with which two checker-chambers 22 and 23'are associated. Numeral 24 indicates a compartment which communicates through the stacks 21 and 28 with the checker chambers, as by passages 25 and 26.

The compartment 24 is provided with two stacks 21 and 28, and these stacks are provided with dampers K and K respectively, having weighted opening levers Ka and Ka'. In normal operation one of these dampers is closed and the other open; and the damper positions are reversed periodically. For supplying air under pressure to the'compartment 24, there is provided one or more fans. In the present instance, two fans, F and F are shown; and these fans Either of the fans may be operated singly, or both fans may be operated simultaneously, depending upon conditions. The usual gas line leading to the furnace is referred to by G, and this 'line is provided with a cut-off valve Ga. This valve must be opened manually'as by a suitable handle such as Gb, but is held in open position, during normal operation of the furnace, by a magnet winding Go and a cooperating magnetic core Gd.

Assuming that the damper K is closed and the damper K open, then the air under pressure supplied to the compartment 24 by either or both fans, cannot escape through the stack 28 and therefore a portion of it passes through the flue 25, the checkers 22, and thence to the furnace; and some of the air under pressure in the compartment will pass out through the stack 21, thereby aiding in ejecting the wastegases drawn through the checkers 23. Adjustable dampers 24f are preferably provided in the'compartment 24 to control the volume flow of air. The above described operation will bereversed, of course, when the dampers'K and K are reversed.

The operation of the forced draft control for regenerative or recuperative furnaces will be understood from the foregoing brief description. Some of the undesirable and possibly dangerous conditions which can arise in the operation of forced draft, regenerative or recuperative furnaces have also been mentioned, and we shall now describe the protective system for preventing such conditions and for giving instant warning;

it being understood that the present invention 7 does not relate in any way to the forced draft regenerative furnace per se.

The two fan motors are supplied with operating current from a three-phase power line indicated at I in which is included a line switch 2; and individual line switches 3 and 3 being pro vided for the respective motors M and M. It will be understood that these line switches will be properly fused to protect the line against abnormal current conditions. Starting switches 4 and 4 are located in the supply circuits of the two motors and are operated respectively by electromagnets 4a and 4a.. A relay C is connected across two of the supply leads to motor M and is energized upon the closing of the starting switch 4. A similar relay C is connected in a like manner to the line supplying motor M.

Current for energizing the protective circuits is derived from one phase of the main power line through a step-down transformer 5 provided with a primary switch Band a secondary switch 1, which are properly fused to protect the circuit from abnormal current conditions. It will be understood that the transformer 5 will not be needed where the voltage of the main line is suitable for the protective circuits. Current for energizing magnets Kb and Kb to operate the stack dampers K and K is supplied from the transformer 5 over conductors '|a1b, relay switch 8, conductors 8b8c, and switch 9. It will be understood that switch 9 is a spring actuated type which therefore has no off position. The magnet winding Go for maintaining the main fuel supply valve open is energized by a circuit connected across conductors 8b and 8c, and the alarm relay E is likewise energized by a circuit connected across these conductors.

The circuit for energizing operating winding 4a of the starting switch 4 is connected across two conductors on the live side of the starting switch 4 and includes, in series, start button A and contact b on run button B. Contact a on run button B is arranged to establish a path in shunt to start button A, through the inter-lock 4b of switch 4, and contact h of alarm relay E establishes a path in shunt to contact b of run button B. A stop button S is interlocked with run button B, and is provided with a contact 0 which establishes a path in shunt to the contact d of relay C. Similar start, run and stop buttons A, B and S with corresponding circuit connections are provided for the motor M' as shown in the drawing. The interlocked push buttons 13-8 and BS constitute push button switches having a run position in which contacts a and a are closed. and a stop po sition in which contacts I), b and c, c are closed. A suitable alarm or signal device, such as an electric horn or howler II, is connected in'cir- 'cuitwith an energizing battery l2, and is controlled by contact 9 on the alarm relay E. The arrangement is such that the alarm is sounded when therelayE is jde-energized. The battery buttons for the motor M and is controlled by contact f of relay 0'.

The circuit diagram shown in the drawing illustrates the position of the various electrically controlled elements when their circuits are deenergized. When it is desired to operate the furnace, switches 2, 3, 3, 6 and I are closed. Also, switch 9 is placed in the proper position to close the damper in the proper stack, for example,

if' air is to be forced through checker chamber 23, the switch 9 is thrown to the left to energize magnet Kb and close the damper K. It will be assumed that during the first stage of operation of the furnace only one fan will be needed and for this purpose it is desired to start fan F. To do this, the following procedure must be followed: Stop buttons S and S are pushed in to the position shown. Start button A is depressed to complete the circuit of the switch winding 4a which has been prepared through contact I) of run switch B. The start button A is held in depressed position until the motor M begins to operate, at which time the relay C will be operated. Relay switch 8 is operated upon the closing of relay C, the circuit for the operating magnet 8a being completed from conductor 1a, through contact d on relay C, contact 0 of stop button S, through the magnet winding, and through contact 6 of relay C to conductor lb. Upon the closing of relay switch 8 the lamp I0 is energized, the circuit having been established through contact 1 of relay C. The signal controlling relay E is also energized and moved to its operating position. With the start button A still depressed, the run button B is next depressed, and then the start button A is released, which completes the operation for starting the motor M. The furnace is now started by manually opening the valve Ga in the supply pipe G and this valve is maintained in open position by the magnet Gc. Also,'the magnet Kb becomes energized upon the closing of relay switch 8 and switch 9, and the closing of damper K forces the air to pass through the checker chamber 23, and into the furnace 2|. A portion of the air supplied to the compartment 24 passes out through the stack 28 for the purpose of drawing the combustion gases out of the furnace 2| and checker chamber 22.

Assumingv that the furnace is operating with only motor M energized and that it is desired to start the other motor M, the following procedure is used: The stop button S is now in the stop position. The start button A is closed to comrun button B is depressed, and the start button A is then released, and the starting operation is complete.

In case the furnace is being operated with only one motor energized, motor M for example, and it is desired to stop the motor, which involves a complete shut-down of the furnace, the following procedure applies: The start button A is depressed, and while still holding the start button depressed, the stop button S is pushed in, and the start button is then released. The opening of the start button deenergizes the starting switch 4, which results in a de-energization of relay C, which in turn opens the circuit of relay 8a and thereby de-energizes the protective oil"- cuit. The solenoid Gc will be de-energized and the valve in the gas supply line will be closed, the howler II will begin to operate, and the damper K will open. It will be noted that with only one motor operating, it is possible to shut down the furnace by merely pushing the stop button of the operating motor, but the above described procedure is recommended as being a procedure which will permit the stopping of one motor while another motor'is running without shutting down the furnace. If two motors are running, the depression of either stop button may shut down both motors.

In case both motors are running. and it is desired to stop only one motor, for'example,.motor M, the following procedure applies: Start button A is depressed and held depressed, then stop button S is depressed and the start button A released. With this procedure, a circuit is maintained through the relay 8a, the relay E remains energized and the circuit of motor M is not disturbed. If the abovesimple procedure is not followed in shutting down one motor, while both are running, both motors may be stopped and the furnace shut down.

It is possible to shut down the furnace by opening either switch 6 or switch I, but this procedure is not recommended except in case of an emergency for the reason that it leaves the motor stop buttons in the wrong position for starting again.

From the foregoing description it will be seen that the control circuit provides the following checks against improper starting of the furnace:

(a) Both stop buttons must be placed in stop position. Otherwise the proper sequence of operation of relays upon starting will not follow, and upon release of the Start button, the motor will cease to operate.

(b) Switches 6 and I must be closed, otherwise the motor will stop when the start but-, ton is released.

(0) If the run button is pushed before the start button, the motor will not start.

(d) With one motor already operating, the

second motor must be started by first pushing the start button and then the run button, otherwise the first motor will be disconnected.

(6) Switch 9 prevents both stack dampers from being left in an ejecting position.

(f) The gas valve Ga. cannot be maintained open unless the protective circuit is energized.

In the case of a complete power failure, which might be due to a loss of power on the main line or a blow-out of the fuses in switch 2, all motors that are running will immediately stop. The starting switches 4, 4' will open, and relays C, C and 8 d will also open. The opening of relay 8a de-energizes relay E and magnets Gc,

Kb and Kb, thus starting the operation of the 'howler ll, closing the fuel supply valve, and

opening the closed stack damper. With the return of power to the system, the fan motors do not automatically pick up, but must be started by the operator. This prevents the fans from pumping air through the furnace without the operators knowledge that the gas has been shut off. [In order to start the furnace operating again, the operator must place the motor stop buttons in the stop. position; press the start button, then press the run button while the start button is still depressed, release start button; and then manually open the gas valve. The damper magnet that was energized when the powe'r failed will be energized immediately when the; protective circuit is re-established, without any attention from the operator. The howler will also stop operating as soon as the protective circuit is again energized. It .is to be noted that the fan motors cannot be operated (although they will run as long as the start buttons are held in) with the protective circuit line switches 6 and 1 open.

With only one motor running, a failure of the motor circuit, as by a blow-out of the fuse in switch 3, or due to any other cause, the switch 4 and relays C and 80. will open, relay E will close the howler circuit and the furnace will be shut down as described above for a power failure when both motors are running. The furnace can be started again by first pushing the stop button of the motor which was running at the time of the circuit failure, then starting the other motor and manually opening the gas valve. 7

When both motors are operating, and there is a failure of one motor circuit, both motors will be stopped and the furnace shut down. Until the defective circuit can be repaired, the other motor can be started again and the furnace operated at a reduced rating with one fan.

Upon a failure of the protective circuit which supplies energizing current to the relay E, to the stack damper magnets and to the valve holding magnet Go, all motors which are operating will be stopped and the furnace shut down as described above. To start the furnace again it is necessary to clear the protective circuit of the ground or short-circuit, or open circuit, insert new fuses, and start the motors as described hereinbefore.

Failure of any operating line switch 4, 4 or of any of the relays C, C, 8a and E, as by a shortcircuit of the operating coil, or by a contact failure, in the control circuit, willstop the motors and shut down the furnace. After replacing the defective coil or repairing the faulty contact, the furnace may be started again as described hereinbefore.

"As previously mentioned, the invention relates to a protective system for forced draft regenerative and rccuperative furnaces, and'inthe claims the definition of a regenerative furnace is, of course, intended to include a recuperative furnace. The invention has been illustrated in connection with a regenerative furnace, and to adapt the protective system to a recuperative furnace it would only be necessary to omit the damper operating circuits.

The invention will be clearly understood from the foregoing description, and the numerous advantages thereof in preventing undesirable and possibly dangerous conditions, will be apparent to those skilled in the art. It will also be apparent to those skilled in the art that while the invention has been described in detail, yet it may be widely changed and modified without departing in any respect from the spirit of the invention; and all such changes and modifications are intended to be included within the scope of the appended claims.

What we claim is:

1. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower,-means for delivering fuel to the furnace, a stack damper, means for closing the stack damper, and means automatically shutting off the fuel supply upon failure of the damper closing means.

2. In combination, 'a .iregenerative furnace, 3

blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a stack damper, means for closing the stack damper, a. signal device, means automatically shutting off the fuel supply upon failure of the damper closing means, and means for effecting the operation of said signal device.

3. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a stack damper, electrical means for closing the stack damper, and means automatically shutting off the fuel supply upon failure of said electrical damper operating means.

4. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a stack damper, means for closing the stack damper, and means automatically shutting down the blower motor upon failure of said damper closing means.

5. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a stack damper, electrical means for closing the stack damper, and means automatically shutting down the blower upon failure of said electrical damper operating means.

6. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, astack damper, electrical means for closing the stack damper, a signal device, means automatically shutting down the blower upon failure of said electrical damper operating means, and means for effecting the operation of said signal device.

7. In combination, a regenerative furnace, a blower for causing .forced draft through the checkers, .a motor for operating the blower, means for delivering fuel to the furnace, a stack damper, electrical means for closing the stack damper, and means automatically shutting down said blower and shutting off the fuel supply upon failure of said electrical damper operating means.

8. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel'to the furnace, a stack damper, electrical means for closing the stack damper, and means for opening said damper and shutting off the fuel supply upon failure of the motor operating means.

9. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor foroperating the blower, means for delivering fuel to the furnace, a valve controlling the fuel supply, :means for maintaining said valve open, a stack damper, electrical means for closing the stack damper, and means for opening the stack damper upon failure of said means for maintaining said valve open.

'10. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, amotor for operating the blower, means for delivering .fuel to the furnace, a valve for controlling the .fuel supply, electrical means for maintaining said valve open, a stack damper,

electrical means for closing the stack damper,

and means .for opening the stack damper upon .means,.and means preventing improper starting ,whenthe furnace is restarted. r 1

.failure of said electrical means vfor maintain- ,ing said valve open.

11.,In combination, a regenerative'furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a valve controlling the fuel supply, electrical means maintaining said valve open, a stack damper, electrical means for closing the stack damper, and means for shutting down said blower upon failure of said electrical means for maintaining said valve open.

12. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating theblower, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means maintaining said valve open, a stack damper, electrical means for closing the stack damper, and means for shutting down said blower and opening the stack damper upon failure of said electrical means for maintaining said valve open.

13. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for operating the blower, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means for maintaining said valve open, a stack damper, electrical means for closing the stack damper, a signal device, means for shutting down said blower and opening the stack damper upon failure of said electrical means for maintaining said valve open, and means for effecting the operation of said signal device.

14. In combination, 'a regenerative furnace, a blower for causing forced draft through the checkers, electrical means for operating the blower, a stack damper, electrical means for maintaining said damper closed, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means for maintaining the valve open, and means for shutting down said blower, opening said stack damper, and shutting off the fuel supply upon failure of any of said electrical means.

15. In combination, a regenerative furnace, .a

blower for causing forced draft through the checkers, electrical means for operating the blower, a stack damper, electrical means .for maintaining said damper closed, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means for .maintaining the valve open, and means for shutting down said blower, opening said stack damper, and shutting off the fuel supply, upon any power failure.

16. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, electrical means for operating the blower, a stack damper, electrical means for maintaining said damper closed, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means for maintaining the valve open, and means for shutting down the furnace upon failure of any of said electrical means, said valve being openable by hand only when said furnace is again started.

1'7. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, electrical means for operating the blower, a stack damper, electrical means for maintaining said damper closed, means for delivering fuel to the furnace, means for shutting off the fuel supply, shutting down said blower and opening said damper upon failure of any of said electrical 18. In combination, a regenerative furnace, a

er, a stack damper, electrical means for maintaining said damper closed, means for delivering fuel to the furnace, means for shutting down the furnace upon failure of any of said electrical means, means preventingimproper starting when the furnace is restarted, and means causing the furnace to be again shut down when not started improper sequence.

.19. In combination, a regenerative furnace, two blowers for causing forced draft through the checkers, two motors for operating the blowers, means for delivering fuel to the furnace, a stack damper, means for maintaining said damper closed, and means for-shutting down both motors upon failure of either motor circuit, shutting off the fuel supply, and opening said stack damper.

20. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, an electric motor for operating the blower, means for delivering fuel to the furnace,

1 holding means and said damper holding means.

' 22. In combinatioma regenerative furnace, a plurality of blowers for causing forced draft through the checkers, a plurality of electric mo tors for operating the blowers, means for delive ering fuel to the furnace, a stack damper, electrical means for maintaining the damper closed, and means responsive to the energization of any of said motors for energizing said damper operating means.

23. In combination, a regenerative furnace, .a plurality of blowers for causing forced draft through the checkers, a plurality of electric mo tors for operating the blowers, means for delivering fuel to the furnace, a valve for controlling the fuel supply, electrical means for maintaining the valve open, a stack damper, electrical means for maintaining the stack damper closed, and means responsive to the energization of any of said electric motors to energize. .said valve holding means and said damper holding means.

24. In combination, a regenerative furnace ,provided with a blower for causing forced draft through the checkers, means .for delivering .fuel

to the furnace, a motor for operating saidblower, a source of current for driving said motor, a line switch for connecting said source to said motor, a stack damper, means for closing said stack damper, and an electric relay connected on the motor side of said line switch to control the operation of said damper operating means.

25. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for driving said blower, means for delivering fuel to the furnace, a valve for controlling the fuel supply, a line switch for connecting said motor to a source of current, a magnet for operating said switch, a circuit for energizing said magnet including push-button switches, means for maintaining said valve in open position, a circuit for energizing said maintaining means, means dependent upon the closing of said line switch and responsive to the normal operating conditions of said motor to energize said valve maintaining circuit, and means responsive to the energization of said valve maintaining circuit for maintaining the continuity of the energizing circuit of said line switch magnet after starting.

26. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for driving said blower, means for supplying fuel to the furnace, a line switch con- .nected to said motor, a magnet for closing said line switch, a start push-button switch biased to open position, a second push-button switch having a run position and a stop position, a circuit for energizing said magnet including in series a contact on said start push-button and a contact in the stop position of the second push-button switch, means responsive to the normal running condition of said motor for establishing a path in shunt to the stop contacts of the second pushbutton switch, and a contact in the run position of said second push-button switch for shunting the contacts of said start switch.

27. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, a motor for driving said blower, means for supplying fuel to the furnace, a line switch connected 'to said motor, a magnet for closing said line switch, a start push-button switch biased to open position, a second push-button switch having a run position and a stop position, a circuit for energizing said magnet including in series a contact on said start push-button and a contact in the stop position of the second push button switch, a relay connected to the motor side of said line switch and adapted to operate in response to the normal running voltageof said motor, means controlled by said relay for shunting the stop contacts of the second push-button switch, and a contact in the run position of said second push-button switch for shunting the contacts of said start switch.

28. In combination, a regenerative furnace, a blower for causing forced draft through the checkers, means for supplying fuel to the furnace, a motor for driving said blower, a line switch for connecting said motor to a source of current, a magnet for operating said switch, a circuit for energizing said magnet including push-button switches, a stack damper, means for operating said stack damper, a circuit for energizing said damper operating means, means dependent upon the closing of said line switch and responsive to the normaloperating conditions of said motor to energize said damper operating circuit, and means responsive to the energization of said damper operating circuit for maintaining the con- *tinuit y of the energizing circuit of said'line switch magnet after starting, whereby a failure of said damper operating circuit will open said line switch.

29. In combination, a regenerative furnace, two 7 blowers for causing forced draft through the checkers, two electric motors for operating said blowers, means for supplying fuel to the furnace, magnetic line switches for each of said motors, a start push-button for each line switch, an auxiliary control switch for each line switch having a run position and astop position, and energizing circuits for each line switch controlled by said start and auxiliary switches whereby in the starting of either motor, both auxiliary switches must be moved to the stop position, the start button depressed, and then the auxiliary switch moved to run position.

30. In combination, a regenerative furnace, two

blowers for causing forced draft through the checkers, two electric motors for operating said blowers, means for supplying fuel to the furnace, a line switch for each motor, a magnet for closing each of said line switches, a start push-button switch for each of said line switches biased to open position, a second push-button switch for each motor having a run position and a stop position, a circuit for energizing each magnet including in series a contact on the start pushbutton and a contact in the stop position of the second push-button switch, and contacts in the run position of said second push button switches for shunting the contacts of said Start switches, a relay connected on the motor side of each line switch and being responsive to the normal operating conditions, of said motors to shunt second contacts in the Stop position of the second push button switch.

31. In combination, a regenerative furnace, two blowers for causing forced draft through the checkers, two electric motors for operating said blowers, means for supplying fuel to the furnace, a line switch for each motor, a magnet for closing each of said line switches, a start push-button switch for each line switch biased to open position, a second push-button switch for each motor having arun position and a stop position, a circuit for energizing each magnet including in series the contact of the start push-button and a stop contact of the second push-button switch, a relay connected on the motor side of each line switch and being responsive to the normal operating conditions of said motors to shunt second contacts in the Stop position of the second push button switch, and a relay whose coil is responsive to a series circuit containing contacts of the aforementioned relays and a circuit controlled by said relay, which controls the energization of said magnets of said motor line switches.

LAURENCE QMEHARG. JACOB H. KAYLOR. DONALD r. RODENBAUGH. 

